20 Abstract Class Interview Questions and Answers

An abstract class is a class that contains one or more abstract methods. An abstract method is a method that is declared, but not implemented in the code. When a class contains an abstract method, the class must also be declared as abstract. Abstract classes cannot be instantiated, meaning an object cannot be created from them. In this article, we will discuss abstract class interview questions.

Abstract Class Interview Questions and Answers

Here are 20 commonly asked Abstract Class interview questions and answers to prepare you for your interview:

1. What is an abstract class in Java?

An abstract class is a class that cannot be instantiated, meaning you cannot create an object of that class type. Abstract classes are meant to be extended by other classes, and they typically contain one or more abstract methods, which are methods that do not have a concrete implementation.

2. Can you explain the purpose of a constructor in an abstract class?

The purpose of a constructor in an abstract class is to initialize the class and its member variables. It is also used to set up any dependencies that the class may have.

3. How can you achieve abstraction with abstract classes?

Abstract classes are one of the ways that you can achieve abstraction in Java. Abstract classes allow you to define a class without having to provide all of the details of how it will be implemented. This means that you can create a general outline of a class, and then let subclasses fill in the details. This can be helpful when you are trying to create a class that will be used in multiple different ways.

4. Is it possible to instantiate an object from an abstract class? If yes, then how?

No, it is not possible to instantiate an object from an abstract class. An abstract class is a class that is meant to be extended, not instantiated.

5. Is it possible to create static methods or variables in an abstract class? If yes, then why do you think they are allowed?

Yes, it is possible to create static methods or variables in an abstract class. This is allowed because static methods or variables are not tied to any particular instance of a class, and so can be used even if no instance of the class exists. This can be useful in situations where you want to provide a general utility method or variable that can be used by any code, regardless of the specific class that it is being called from.

6. What’s the difference between an interface and an abstract class?

An interface is a contract that a class can choose to implement. An abstract class is a class that contains abstract methods, which must be implemented by any child classes.

7. Why would you want to use an abstract method instead of an interface?

An abstract method is a method that is declared, but not necessarily implemented, in a class. An interface, on the other hand, is a completely abstract class that can only contain abstract methods. The main advantage of using an abstract method is that it allows you to partially implement a class, which can be helpful if you want to provide a default implementation that can be overridden by subclasses.

8. When should we prefer using interfaces over abstract classes?

When you want to specify the behavior of a class without providing any implementation, you would use an interface. An abstract class, on the other hand, allows you to provide some implementation along with the ability to specify behavior. So, if you want to provide a partial implementation of a class, or if you want to be able to specify behavior and also provide some implementation, then you would use an abstract class.

9. Do all methods declared in an interface need to be implemented by derived classes?

No, not all methods declared in an interface need to be implemented by derived classes. However, any methods that are not implemented will need to be declared as abstract methods in the derived class.

10. Which one do you think provides better code reusability – abstract classes or interfaces?

It really depends on the situation. If you need to provide a lot of common functionality between different classes, then an abstract class might be a better choice. However, if you only need to specify certain behavior that must be implemented, then an interface might be a better fit.

11. Why is it important to have constructors defined in abstract classes?

Constructors are important in abstract classes because they allow you to initialize member variables of the class. If you do not have a constructor defined in your abstract class, then the member variables will not be initialized and you will not be able to use them.

12. Why do you think it’s not possible to declare abstract constructors?

Abstract constructors are not possible because they would never be called. The purpose of an abstract constructor is to provide a template for child classes to follow when they are creating their own constructors. However, since the child classes would never actually call the abstract constructor, there would be no point in having one.

13. What do you understand about encapsulation?

Encapsulation is the process of hiding data within an object so that it is not directly accessible from outside the object. This helps to protect the data and keep it from being tampered with.

14. What are some best practices that you follow when designing an abstract class?

When designing an abstract class, I always make sure to include both abstract and concrete methods. This allows for a balance between flexibility and functionality, and ensures that the abstract class can be used in a variety of different ways. I also make sure to document the abstract class thoroughly, so that other developers will be able to understand how it is meant to be used.

15. What’s your opinion on having final classes extend an abstract class?

I think that it can be a good idea to have final classes extend an abstract class because it can help to enforce a certain level of consistency and organization in the code. It can also help to make sure that all of the necessary methods and fields are implemented in the child class.

16. Can you explain what polymorphism means in Java?

In Java, polymorphism refers to the ability of an object to take on multiple forms. This means that an object can be created that can be used as a superclass, but can also be used as a subclass, depending on the context. This is possible because Java uses something called dynamic binding, which allows for different methods to be called on an object depending on the context in which it is being used.

17. Are there any limitations imposed on abstract classes that don’t apply to interfaces?

Abstract classes can contain both abstract and concrete methods, while interfaces can only contain abstract methods. Abstract classes can also contain fields and static methods, while interfaces cannot.

18. What kind of operations require overriding?

In order for an operation to require overriding, it must be declared as abstract in the base class. If an operation is not declared as abstract, then it will not require overriding in the derived class.

19. What do you understand about abstraction?

Abstraction is the process of hiding the implementation details of a class or method from the user. By doing this, it allows the user to focus on the functionality of the class or method, without needing to worry about the underlying code. This can be helpful in making code more readable and easier to understand.

20. Can you give me some examples of where you might encounter abstract classes in real-life applications?

Abstract classes are often used in frameworks to provide a basic template that can be extended by other developers. For example, in the Java programming language, the AbstractWindow class provides a basic template for creating a window that can be further customized. Other examples might include an abstract Shape class that could be extended to create specific shapes like circles, squares, etc.